Incremental printing is based on accurate deposition of small colorant dots onto specified locations on paper or other printing media. In inkjet printing such placement takes the form of ballistic delivery of ink droplets.
Typically these mechanisms form a rectangular grid of specified resolution, the most common resolutions now being twelve by twelve, or twenty-four by twenty-four, dots per millimeter (three hundred by three hundred or six hundred by six hundred dots per inch). Other formats, however, are continuously evaluated.
At least two important mechanisms give rise to intractable difficulties in the control of CDE. As to the types of CDE associated with dot-density variations, such stringent difficulties occur even in monochrome printing.
As to the types of CDE associated with optimum print-medium-advance variations, such difficulties generally exceed available correction resources in printing that combines different color planes, most-commonly primary colors but also other color sets such as hexachrome or light colors form the images. In this case the major difficulty arises directly from the basic requirement for interrelated delivery of different colorants into common areas.
1. Error Types
For purposes of this document, CDE encompasses at least four main types of directly observable error—each of which can occur alone under some conditions, although these types are generally interrelated in complex ways:                (1) individual-element density error,        (2) swath-height error (“SWE”),        (3) area-fill nonuniformity (“AFNU”), and        (4) ink-media interactions.        
The first of these refers to individual printing elements—whether or not correctly aimed—whose printed dots are either too dark or too light. In inkjet printing such error can be due to variation in drop weight, drop shape or other effects.
The second, SWE, refers to swaths that appear too tall or too shallow, most commonly regarded as due to aiming errors near the ends of the array. Some SWE effects, however, can result from density errors in those regions. (The acronym “SWE” derives from earlier popular nomenclature, “swath width error”.)
The third type of error, AFNU, refers to nonuniform density in an image field that is printed in response to uniform image-data. This kind of error can result from either of the first two errors—or from aiming error not particularly concentrated at the array ends, or from an undefined complex of dot-placement attributes.
Such placement attributes most likely implicate interactions between colorant and a printing medium on which the colorant is deposited. This is the fourth category of error effects—“ink-media interactions”.
(The terminology AFNU, here “area fill nonuniformity”, is used in some industrial facilities to connote a more-specific type of defect—a blotchy or mottled appearance. The present inventors wish to point this out simply to avoid confusion due to these slightly different usages. AFNU as used in this document may be regarded as meaning in essence “swath fill nonuniformity”.)
The effects and causes discussed above are not related to each other in rigorously the cause-and-effect ways suggested. Thus for example a cause of the third type of error, nonuniform density, can be ink-media interactions; and such interactions, for some purposes, accordingly might be better listed as a cause, rather than an effect. As will be seen shortly, precise categorization of these relationships is not significant to either understanding or validity of the present invention.
While AFNU and SWE may present themselves to a viewer as distinct matters of spatial distribution and spatial deformation respectively, in actuality what appears to be a deformation of swath height (or any other shape) can be caused by perturbed colorant distribution. In other words deformation is nested within distribution error.
2. Shortened Life of Printing Arrays
Currently multielement printing arrays (including for example “printheads” or multinozzle “pens” in inkjet printing) are discarded when they develop serious problems of any of these types—although attempts have been made to deal with SWE in particular by simply accommodating such error through modification of the distance or stroke of printing-medium advance. Premature discarding of printing arrays is very undesirable because it directly elevates the end user's operating cost.
3. Modification of Printing-Medium Advance
Trying to accommodate SWE by modifying the stroke, without more, has drawbacks. Among these perhaps the most prominent is that such modification lengthens or shortens the overall dimension of the image, in the advance direction—and accordingly in some cases the dimension of the printed sheet. This makes impossible, in general, the piecing of several images together in a regular, tidy mosaic to make a large composite image.
In plural-color printing systems, another drawback is that each different color is associated with a respective different printing array and therefore, in general, with a different SWE—requiring, in turn, a different modified stroke. Only one stroke value is possible for the overall plural-array system; hence stroke modification cannot accommodate the height errors for all the colors.
As suggested above, SWE—with a resulting banding appearance—is one particularly conspicuous consequence of inaccurate dot placement, i. e. aiming error. Placement inaccuracy also causes other forms of banding, as well as line discontinuity and roughness, and color anomalies.
There are several contributors to dot-placement inaccuracies. Some of these arise in the multielement printing arrays, and others in other portions of the printing apparatus.
Such inaccuracies can occur along the scan axis (in scanning systems) or the printing-medium advance axis, or both. Some are systematic, while some others follow random patterns.
As to aiming errors, this document focuses upon the systematic component of those errors which lies along the advance axis. A typical source of these particular aiming-error components is advance-axis directionality of individual elements in the printing array.
In inkjet printing, such misdirected elements in turn can be due to relative misalignments between an array of firing resistors (or “heaters”) and an array of nozzle orifices (or “nozzle plate”). Such defects, though tiny, cause drop-ejection directionality in both the scan (when applicable) and advance axes, the latter being a particular concern of the present invention.
When manifested as SWE, these defects generate a difference h (FIG. 10) between nominal printhead height H and the actual printed swath height H+h. As the left-hand and right-hand views demonstrate, the error h—identifiable as the quantitative SWE—can be either positive (h>0) or negative (h<0, H+h<H). The center view shows the nominal condition in which the error h is zero, i. e. there is no error.
Generally, techniques of accommodating SWE by adjusting the advance stroke start with assumption of some model that explains observed banding in terms of the SWE and the stroke; such a model in effect establishes a relation between the error and the stroke.
The problem can be made more specific with an example. In attempting to print a uniform area fill (FIG. 11, left-hand view) with one printing array (printhead) in a single-pass mode, the system advances the medium—between successive passes—by a stroke equivalent to the nominal array height H.
If the printhead has a negative SWE (center view), however, then adjacent swath fail to abut; this failure leaves white streaks between consecutive swaths. Such artifacts will be called “white-streak banding”.
On the other hand, if the head has a positive SWE (right-hand view) then adjacent swaths overlap; the printed image in the overlap regions appear darker. Artifacts of this second kind will be called “dark banding”. As the illustrations make plain, both cases represent a large adverse impact on print quality.
Another typical source of image banding is inaccuracy in the print-medium advance mechanism. Again assuming an ideal uniform fill (FIG. 12 left-hand view), if the medium underadvances, the image contains dark banding (center view)—similar to the appearance discussed above for positive SWE.
If the medium overadvances, then what appears instead is white-streak banding (right-hand view) like that noted above for negative SWE. Either kind of advance error accumulates, so that the overall length of the printed image varies in proportion to the amount of under- or overadvance (h per pass, times a number of passes); whereas with SWE the overall image length varies only by an amount equivalent to one times h, independent of the number of passes.
Now with such a model providing a theoretical relation between SWE and stroke, prior efforts to accommodate SWE include adapting the stroke to the actual effective swath height, or in other words to take into account the SWE. Again comparing with an ideal case of zero SWE, zero stroke adjustment (FIG. 13, left-hand view), a negative SWE is accommodated by a matching stroke reduction (center view) so that the white-streak-separated swaths of FIG. 12 are much more nearly abutted and the overall fill appears much more neatly blended.
A positive SWE, conversely, is accommodated by a matching stroke increase (FIG. 13, right-hand view), so that the overlapping swaths are moved apart to very nearly just abut and again the overall fill appears much more neatly blended. Unfortunately neither increase nor reduction of the stroke can work for more than one print array at a time, if—as is generally the case—the arrays have different effective swath heights (FIG. 14).
For any specified image, however, the stroke can be adjusted to equal some sort of balanced or weighted mean of the several swath heights. This balance can take into account which color is used most in the swath, to minimize banding in that color plane.
To accomplish that, the stroke can be instead adjusted to the actual swath height of the printhead which is used most—in the specific corresponding image data file (either overall or swath by swath)—as taught by Doval, mentioned earlier. Still, when two colors must be used in equal proportions, the best that can be done is only accommodation for the average SWE.
It will be understood from the foregoing that the system need store only a very modest amount of data to accomplish these tasks. More specifically, it may be desired typically to store—for each printhead—both the effective swath height (or some parameter closely related) and the ink usage for a current swath.
In addition it is desired to store the resultant balanced- or weighted-mean swath height—i. e., one additional single number. Hence the totality of data storage for this purpose may be equivalent to a numerical tabulation that has only, say, a number of rows that equals the number of printheads—and two columns (one for effective swath height and the other for current-swath ink usage)—plus the weighted mean.
The number of printheads and therefore rows in the equivalent tabulation is nowadays most typically four, though systems with six or seven printheads are becoming common. In any event the size of the equivalent tabulation, at least currently, is less than ten by two, plus the resultant weighted swath-height value (again, just one single number).
In the course of calibration, and preparation for operation, the system in effect modifies a tabulation of this general size. The rough size of this tabulation or data array may be borne in mind for comparison with later discussions of the invention.
4. Automatic Substitutions and Weighting
As to density error, a current tactic substitutes healthy printing elements for defective ones—either directly or on a statistical, weighting basis—as is taught, for instance, in the above-mentioned earlier patent documents of Garcia-Reyero. This approach, however, has its own distinct limitations.
It requires use of multipass printmodes, which is relatively slow. If many elements behave poorly, this approach may not work or may require switching to an even slower printmode.
The weighting versions of this technique are more broadly applicable, for they allow defective nozzles to be used less than healthy ones—rather than not at all—and thereby tend to make whatever use can be made of each nozzle. As a practical matter weighting appears to be more useful in cases of misdirected elements than weak or overstrong elements.
Density errors due to elements that form too-dark or too-light marks are not corrected adequately by any prior technique—particularly not any that is usable with a small number of passes, e. g. one- or two-pass printmodes. The same is true of ink-media interactions; and the foregoing discussions also cover AFNU, whether associated with SWE or density phenomena.
As is well known, an incremental printing system establishes average density levels through processes called “rendition”, which most typically take the form of either dithering or error diffusion. Dithering employs a relatively large dither mask or rendition matrix—a much larger numerical data tabulation than the effective tabulation discussed above as to SWE management.
The dither mask is substantially greater, ordinarily, than a ten-row-by-ten-column table; however, it is set at the factory and ordinarily undergoes no modification in the field. This too may be borne in mind for comparison with later discussion of the invention.
5. Cost
Furthermore, these several limitations of corrective techniques known heretofore are present even though multielement printing arrays are subject to relatively stringent manufacturing tolerances and therefore relatively expensive. Manufacture and use of printing arrays (inkjet pens etc.) could be considerably more economical if the printing apparatus and methods were significantly more tolerant of both aiming and density errors, as well as ink-media artifacts.
6. Conclusion
Inadequate management of the four main error types introduced above has continued to impede achievement of uniformly excellent incremental printing—at high throughput—on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.